This invention relates to a cementitious composition comprising a hydraulic cement and water and to a cementitious product produced by setting of the cementitious composition, and in particular it relates to a cementitious product having a high strength.
Cementitious products made by setting of a cementitious composition comprising a hydraulic cement and water are generally of low strength, and in particular generally have a low flexural strength as measured, for example, by the three-point bend flexural strength test. For many applications, and in particular where such products must withstand high flexural loads, it is desirable that the flexural strengths of such products be increased. A known method of improving the flexural strength of a cementitious product is to add fibrous material to the cementitious composition from which the product is produced. For example, it is known to increase the flexural strengths of cementitious products from a value of aproximately 5 MPa to a value of approximately 40 MPa by addition of fibrous material to the cementitious composition, as described by D. J. Hannant, Fibre Cements and Concretes (Wiley, London 1978). Fibrous materials which are used to increase the flexural strengths of cementitious products include asbestos, glass, steel, ceramic, polymer and vegetable fibrous materials.
The use of fibrous material however suffers from certain disadvantages. Thus, only relatively low flexural strengths can be achieved in the cementitious products, the flexural strengths rarely exceeding 40 MPa, the presence of the fibrous material in the cementitious composition may adversely affect the rheology of the composition making the composition more difficult to shape into a desired form, the fibrous material may lead to an anisotropic improvement in the flexural strength of the cementitious product, and the fibrous material may be toxic and may be associated with handling difficulties, as in the case of asbestos.
Cementitious products of high strength, and in particular of high flexural strength, are described in European Patent Application No. 80301909.0 in the name of Imperial Chemical Industries Limited, now published as publication No. 0 021 682. In this application there is described a cementitious product produced by setting of a composition comprising a mixture of a hydraulic cement and water not more than 2% of the total volume of the product comprising pores having a maximum dimension exceeding 100 microns, preferably 50 microns, and more preferably 15 microns. Cementitious products which satisfy the aforementioned porosity criteria are of high strength, particularly high flexural strength, and in general the flexural strengths of the products are in excess of 50 MPa. In general, the lower is the proportion of the total volume of the cementitious product comprising pores having a maximum dimension exceeding 100 microns, preferably 50 microns, more preferably 15 microns, the greater is the strength of the cementitious product, and for this reason it is preferred that not more than 0.5% of the total volume of the cementitious product comprises pores of maximum dimension exceeding 15 microns.
We have now found that a high strength cementitious product, particularly a cementitious product with high flexural strength, generally in excess of 40 MPa, may be produced by setting of a cementitious composition without the necessity of using fibrous material in the composition, and furthermore that the product may contain a proportion of pores of size in excess of 100 microns which exceeds the limit of 2% by volume specified in the aforementioned European Patent Publication No. 0 021 682.
However, where the cementitious product produced by setting of the cementitious composition does possess the aforementioned porosity criteria the flexural strength of the product may be greater than, and may even be considerably greater than, the flexural strengths of the products described in the aforementioned publication.